Resonant Converters

ABSTRACT

A resonant tank comprises a resonant inductor coupled to a switching network and a transformer, a resonant capacitor coupled to the switching network and the transformer, a first parallel inductor implemented as a magnetizing inductance and a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor.

TECHNICAL FIELD

The present invention relates to resonant converters, and more particularly, to a full bridge inductor-inductor-capacitor (LLC) resonant converters.

BACKGROUND

A telecommunication network power system usually includes an AC-DC stage converting the power from the AC utility line to a 48V DC distribution bus and a DC-DC stage converting the 48V DC distribution bus to a plurality of voltage levels for all types of telecommunication loads. Both stages may comprise isolated DC-DC converters. Isolated DC-DC converters can be implemented by using different power topologies, such as flyback converters, forward converters, half bridge converters, full bridge converters and the like. As known in the art, bridge converters generally are employed when the power of a DC-DC converter is more than 100 watts.

FIG. 1 (prior art) illustrates a conventional full bridge converter having a full wave rectifier coupled to a center-tapped secondary winding. The full bridge converter 100 includes four switches Q1, Q2, Q3 and Q4 at a primary side of a transformer Tx. The four switches Q1, Q2, Q3 and Q4 form a bridge having two legs. Q1 and Q3 in series connection have a common node, referred to as A. Q2 and Q4 in series connection have a common node, referred to as B. The primary winding of the transformer Tx is connected to A and B. A dc supply Vin is connected to the two legs to provide power to the full bridge converter 100.

According to the operating principle of a hard switching full bridge converter, the switches Q1 and Q4 are turned on simultaneously for an adjustable time during a first half cycle. After a period of dead time, the switches Q2 and Q3 are turned on simultaneously for an equal time during the second half cycle. As a result, Vin and −Vin are applied to the primary side of the transformer Tx in alternate half periods.

In a fixed duty cycle control scheme, the turn-on time of the switches Q1 and Q4 is equal to the turn-on time of the switches Q2 and Q3. When all four switches are turned off, both S1 and S2 are turned on. The load current flows through S1 and S2. This interval is referred to as a freewheeling period. The output voltage of the bridge converter 100 is proportional to the turn-on time of the switches. A controller (not shown) may detect the output voltage Vo and adjust the turn-on time via a negative feedback control loop (not shown).

The secondary side of the transformer Tx is center-tapped. Such a center-tapped secondary and two switches S1 and S2 can form a full wave rectifier, which can convert the primary voltage having double polarities (Vin and −Vin) of the transformer Tx to a secondary voltage having a single polarity. Then, the secondary voltage having a single polarity is fed to an output filter including an inductor Lo and an output capacitor Co. The output filter averages the square voltage pulses at the output of the full wave rectifier and generates a DC voltage at Vo, which is then supplied to a load represented by a resistor R_(L).

A phase shift full bridge converter is capable of reducing switching losses by means of the zero voltage switching control technique. As shown in a dashed rectangle 120 of FIG. 1, instead of turning on two primary switches (e.g., Q1 and Q4) simultaneously, the turn-on time of these two switches are shifted by a period of time. More particularly, as depicted in the dashed rectangle 120, a waveform 106 and a waveform 110 show Q1 is on for a period of time before Q4 is turned on. There is an overlap between Q1's turn-on time and Q4's turn-on time. After Q1 is turned off, Q4 stays on for a period of time. Likewise, a waveform 107 and a waveform 111 show there is a phase shift between Q2 and Q3's turn-on time.

The phase shift full bridge can achieve zero voltage switching by utilizing the L-C resonance between transformer leakage inductance and MOSFET (e.g., Q1) output capacitance. For example, Q3 has a parasitic capacitor (not shown) across its drain and source. During the period when both Q1 and Q4 are on, the voltage across Q3's parasitic capacitor is charged to a voltage approximately equal to Vin. According to the basic principle of the phase shift control technique, Q1 is off prior to Q4. After Q1 is off, the primary side current cannot change instantaneously. As a result, the primary side current will flow through the parasitic capacitors of Q1 and Q3. The flow of the primary side current through both parasitic capacitors may cause the voltage at the junction between Q1 and Q3 to be discharged to zero, enabling zero voltage switching when Q3 is turned on without substantial power losses. Similarly, the phase shift operation may enable a lossless turn-on process of other switches, namely Q1, Q2 and Q4.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide a system, apparatus and method for improving efficiency of an inductor-inductor-capacitor (LLC) resonant converter.

In accordance with an embodiment, an apparatus comprises a resonant inductor coupled to a switching network and a transformer, a resonant capacitor coupled to the switching network and the transformer, a first parallel inductor implemented as a magnetizing inductance and a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor.

In accordance with an embodiment, a system comprises an input power source, a first pair of switches coupled between the input power source, a second pair of switches coupled between the input power source, a resonant tank, a rectifier coupled to a secondary side of the transformer and an output filter coupled to the rectifier

The resonant tank is connected in series with a primary side of a transformer. A parallel inductor of the resonant tank is of an inductance less than a magnetizing inductance of the transformer. A first common node of the first pair of switches is coupled to a first terminal of the resonant tank. A second common node of the second pair of switches is coupled to a second terminal of the resonant tank.

In accordance with another embodiment, a converter comprises a switching network coupled to a dc input power source, a transformer providing isolation between a primary side and a secondary side of the converter, a resonant tank, a rectifier coupled to the secondary side and an output filter coupled to the rectifier.

The resonant tank is coupled to the switching network, wherein the resonant tank comprises a resonant inductor coupled to the switching network and the transformer, a resonant capacitor coupled to the switching network and the transformer, a first parallel inductor implemented as a magnetizing inductance and a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor.

An advantage of a preferred embodiment of the present invention is reducing the switching losses of an LLC resonant converter so as to improve the LLC resonant converter's efficiency.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a full bridge converter having a full wave rectifier coupled to a center-tapped secondary winding;

FIG. 2 illustrates a block diagram of an LLC resonant converter in accordance with various embodiments of the present disclosure;

FIG. 3 illustrates a schematic diagram of the resonant tank shown in FIG. 2 in accordance with various embodiments of the present disclosure;

FIG. 4 illustrates another configuration of the resonant tank shown in FIG. 2 in accordance with various embodiments of the present disclosure;

FIG. 5 illustrates a schematic diagram of the LLC resonant converter shown in FIG. 2 in accordance with various embodiments of the present disclosure;

FIG. 6 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 5 in accordance with various embodiments of the present disclosure;

FIG. 7 illustrates key switching waveforms of the LLC resonant converter shown in FIG. 5 in accordance with various embodiments of the present disclosure;

FIG. 8A illustrates another schematic diagram of the LLC resonant converter shown in FIG. 2 in accordance with various embodiments of the present disclosure;

FIG. 8B illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 8A in accordance with various embodiments of the present disclosure;

FIG. 9 illustrates a schematic diagram of another LLC resonant converter in accordance with various embodiments of the present disclosure;

FIG. 10 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 9 in accordance with various embodiments of the present disclosure;

FIG. 11 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure;

FIG. 12 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 11 in accordance with various embodiments of the present disclosure;

FIG. 13 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure;

FIG. 14 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 13 in accordance with various embodiments of the present disclosure;

FIG. 15A illustrates another schematic diagram of the LLC resonant converter shown in FIG. 13 in accordance with various embodiments of the present disclosure;

FIG. 15B illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 15A in accordance with various embodiments of the present disclosure;

FIG. 16 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure;

FIG. 17 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 16 in accordance with various embodiments of the present disclosure;

FIG. 18 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure; and

FIG. 19 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 18 in accordance with various embodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferred embodiments in a specific context, namely a full bridge inductor-inductor-capacitor (LLC) resonant converter. The invention may also be applied, however, to a variety of converters including half bridge LLC resonant converters, push-pull LLC resonant converters and the like.

FIG. 2 illustrates a block diagram of an LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 200 is coupled between an input dc power source 104 and a load 112. The input dc power source 104 may be telecommunication power supplies converting a utility line voltage to a dc voltage. Alternatively, the input dc power source 104 may be a solar panel array. Furthermore, the input dc power source 104 may be an energy storage device such as rechargeable batteries, fuel cells and/or the like. The load 112 represents the power consumed by a circuit coupled to the LLC resonant converter 200. Alternatively, the load 112 may refer to downstream converters coupled to the output of the LLC resonant converter 200.

The LLC resonant converter 200 may comprise a switch network 108, a resonant tank 109, a transformer 152, a rectifier 154 and an output filter 156. As shown in FIG. 2, the switch network 108, the resonant tank 109, the transformer 152, the rectifier 154 and the output filter 156 are coupled to each other and connected in cascade between the input dc power source 104 and the load 112.

The switch network 108 may comprise primary side switches of a full bridge resonant converter according to some embodiments. Alternatively, the switch network 108 may be of the primary side switches of other bridge converters such as a half-bridge resonant converter, a push-pull resonant converter and the like. The detailed configuration of the switch network 108 will be described below with respect to FIG. 5.

The resonant tank 109 may be implemented in a variety of ways. For example, the resonant tank 109 may comprise a first series resonant inductor, a first series resonant capacitor, a first parallel resonant inductor and a second parallel resonant inductor. The first series resonant inductor and the second parallel resonant inductor may be implemented as external inductors. The first parallel resonant inductor may be implemented as a magnetizing inductance. The configuration of the resonant tank 109 described above is merely an example. There may be many variation, alternatives and modifications. For example, the first series resonant inductor may be implemented as a leakage inductance of the transformer 152.

The resonant tank described above includes three inductors and a capacitor. According to some embodiments, the first parallel resonant inductor is a magnetizing inductance. The second parallel resonant inductor is an external inductance. The first parallel resonant inductor is of an inductance greater than the second parallel resonant inductor. As a result, the second parallel resonant inductor is a dominant factor of the resonant tank 109.

In sum, the resonant tank 109 includes three key resonant elements, namely the first series resonant inductor, the first series resonant capacitor and the second parallel resonant inductor. Such a configuration is commonly known as an LLC resonant converter. According to the operating principle of LLC resonant converters, the resonant tank 109 helps to achieve zero voltage switching for the primary side switching elements and zero current switching for the secondary side switching elements.

The LLC resonant converter 200 may further comprise a transformer 152, a rectifier 154 and an output filter 156. The transformer 152 provides electrical isolation between the primary side and the secondary side of the LLC resonant converter 200. In accordance with an embodiment, the transformer 152 may be formed of two transformer windings, namely a primary transformer winding and a secondary transformer winding. Alternatively, the transformer 152 may have a center tapped secondary so as to have three transformer windings including a primary transformer winding, a first secondary transformer winding and a second secondary transformer winding. It should be noted that the transformers illustrated herein and throughout the description are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the transformer 108 may further comprise a variety of bias windings and gate drive auxiliary windings.

The rectifier 154 converts an alternating polarity waveform received from the output of the transformer 152 to a single polarity waveform. When the transformer 152 is of a center tapped secondary, the rectifier 154 may be formed of a pair of switching elements such as n-type metal oxide semiconductor (NMOS) transistors. Alternatively, the rectifier 154 may be formed of a pair of diodes. On the other hand, when the transformer is of a single secondary winding, the rectifier 154 may be a full-wave rectifier coupled to the single secondary winding of the transformer 152.

Furthermore, the rectifier 154 may be formed by other types of controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like. The detailed operation and structure of the rectifier 154 are well known in the art, and hence are not discussed herein.

The output filter 156 is used to attenuate the switching ripple of the LLC resonant converter 200. According to the operation principles of isolated dc/dc converters, the output filter 156 may be an L-C filter formed by an inductor and a plurality of capacitors. One person skilled in the art will recognize that some isolated dc/dc converter topologies such as forward converters may require an L-C filter. On the other hand, some isolated dc/dc converter topologies such as LLC resonant converters may include an output filter formed by a capacitor. One person skilled in the art will further recognize that different output filter configurations apply to different power converter topologies as appropriate. The configuration variations of the output filter 156 are within various embodiments of the present disclosure.

FIG. 3 illustrates a schematic diagram of the resonant tank shown in FIG. 2 in accordance with various embodiments of the present disclosure. The resonant tank 109 comprises four terminals, namely T1, T2, T3 and T4. There may be a resonant inductor Lr connected in series with a resonant capacitor Cr. The series connected Lr and Cr may be coupled between the terminals T1 and T3.

In addition, there may be a parallel inductor Lp connected in series with a parallel capacitor Cp. As shown in FIG. 3, the series connected Lp and Cp may be coupled between the terminals T1 and T2. It should be noted that the parallel capacitor Cp is an optional element for the resonant tank 109. The resonant tank 109 may function correctly without having the parallel capacitor Cp. However, in some applications such as a full bridge converter, the parallel capacitor Cp may function as a dc blocking capacitor. The dc block capacitor helps to achieve a balanced magnetic flux. As a result, some failures such as saturation of magnetic components can be prevented.

It should be noted that the resonant inductor Lr and the resonant capacitor Cr are similar to the resonant inductors and capacitors employed in conventional LLC converters. The parallel inductor Lp is used to replace the magnetizing inductance of the transformer during an L-C resonant process. In other words, the magnetizing inductance of the transformer 152 is not a dominant element of the resonant process. As a result, the design of the magnetizing inductance may be simplified. For example, the magnetizing gap of the transformer 152 can be omitted while maintaining zero switching such as a zero voltage transition of the primary side switches and a zero current transition of the secondary side switches.

FIG. 4 illustrates another configuration of the resonant tank shown in FIG. 2 in accordance with various embodiments of the present disclosure. The switching network 108 may be formed of a variety of power converter topologies such as full bridge converters, half bridge converters, push-pull converters and/or the like. In addition, some elements of the resonant tank 109 may be moved into the switching networks 108. For example, as shown in FIG. 4, the resonant capacitor Cr shown in FIG. 3 may be replaced by two capacitors of a half bridge converter. The series connected capacitors of a half bridge converter may function as a resonant capacitor, which is coupled to the resonant inductor Lr (shown in FIG. 3) to achieve a zero voltage transition for primary side switches and a zero current transition for secondary side switches.

FIG. 5 illustrates a schematic diagram of the LLC resonant converter shown in FIG. 2 in accordance with various embodiments of the present disclosure. The switch network 108 includes four switching elements, namely Q1, Q2, Q3 and Q4. As shown in FIG. 5, a first pair of switching elements Q1 and Q2 are connected in series. A second pair of switching elements Q3 and Q4 are connected in series. The common node of the switching elements Q1 and Q2 is coupled to a first input terminal T1 of the resonant tank 109. Likewise, the common node of the switching elements Q3 and Q4 is coupled to a second input terminal T2 of the resonant tank 109.

The switching elements Q1, Q2, Q3 and Q4 form a primary side switching network of a full bridge converter. According to some embodiments, switching elements Q1, Q2, Q3 and Q4 are implemented as MOSFET or MOSFETs connected in parallel.

According to alternative embodiments, the primary switches (e.g., switch Q1) may be an insulated gate bipolar transistor (IGBT) device. Alternatively, the primary switches can be any controllable switches such as integrated gate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO) devices, silicon controlled rectifier (SCR) devices, junction gate field-effect transistor (JFET) devices, MOS controlled thyristor (MCT) devices, gallium nitride (GaN) based power devices and/or the like. The detailed operation principles of the switch network 108 will be described below with respect to FIG. 7.

It should be noted that while the example throughout the description is based upon a full bridge LLC resonant converter (e.g., full bridge LLC resonant converter shown in FIG. 5), the LLC resonant converter 200 shown in FIG. 2 may have many variations, alternatives, and modifications. For example, half bridge converters, push-pull converters may be alternatively employed. The full bridge resonant converter illustrated herein is limited solely for the purpose of clearly illustrating the inventive aspects of the various embodiments. The present invention is not limited to any particular power topology.

FIG. 5 shows that the resonant tank 109 is formed by the resonant inductor Lr, the resonant capacitor Cr and the parallel inductance Lp. As shown in FIG. 5, the resonant inductor Lr and the resonant capacitor Cr are connected in series and further coupled to one terminal of the primary side of the transformer 152. The parallel inductance Lp is connected between the first input terminal T1 and the second input terminal T2 of the resonant tank 109 and further coupled to the common node of Q1 and Q2, and the common node of Q3 and Q4.

In order to ensure that the parallel inductor Lp replaces the magnetizing inductance Lm during a resonant process, the magnetic inductance Lm is of an inductance value greater than the parallel inductor Lp. The greater inductance value of the magnetizing inductor prevents the magnetizing current from flowing into the magnetizing inductance Lm. Instead, the magnetizing current flows into a low impedance path provided by the parallel inductor Lp. In some embodiments, the magnetizing inductance is at least ten times greater than the parallel inductance.

The parallel inductance Lp may be utilized in combination with Lr and Cr to achieve zero voltage switching for the primary side switching elements (e.g., Q1) and zero current switching for the secondary side switching elements (e.g., Q8). One advantageous feature of the LLC resonant converter shown in FIG. 5 is that the LLC resonant converter may operate at a frequency close to the resonant frequency of the resonant tank 109. As such, the LLC resonant converter can achieve zero voltage switching for the primary side switches and zero current switching for the secondary side switches. The soft switching operation of the primary and secondary switches helps to achieve high efficiency.

It should be noted while FIG. 5 shows the resonant inductor Lr is an independent component, the resonant inductor Lr may be replaced by the leakage inductance of the transformer 152. In other words, the leakage inductance (not shown) may be utilized as part of the resonant inductor Lr.

The transformer 152 may be of a primary winding and a secondary winding. The primary winding is coupled to terminals T3 and T4 of the resonant tank 109 as shown in FIG. 5. The secondary winding is coupled to the load 112 through a full-wave rectifier 154. It should be noted the transformer structure shown in FIG. 5 is merely an example. One person skilled in the art will recognize many alternatives, variations and modification. For example, the transformer 152 may be a center tapped transformer coupled between the primary side switching network and the secondary side switching network. The secondary side employs a synchronous rectifier formed by two switching elements. The operation principle of a synchronous rectifier coupled to a center tapped transformer secondary side is well known, and hence is not discussed in further detail herein.

It should be noted that the power topology of the LLC resonant converter may be not only applied to a full-wave rectifier as shown in FIG. 5, but also applied to other secondary configurations, such as voltage doubler rectifiers and current doubler rectifiers.

FIG. 6 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 5 in accordance with various embodiments of the present disclosure. In an equivalent circuit, the transformer windings can be removed and various parameters of the secondary can be reflected back to the primary side of the transformer 152. For example, as shown in FIG. 6, the secondary magnetizing inductance is reflected to the primary side. The equivalent inductance of the secondary magnetizing inductance is equal to the secondary magnetizing inductance times n², wherein n is the transformer ratio of the transformer 152. Likewise, the load may be reflected to the primary side. The equivalent load is equal to the secondary load times n.

Based upon the equivalent circuit shown in FIG. 6, the equivalent magnetizing inductance may be described as follows:

L _(m) =L _(m) _(—) _(pri) +n ² ·L _(m) _(—) _(sec)

where L_(m) _(—) _(pri) is the primary magnetizing inductance and L_(m) _(—) _(sec) is the secondary magnetizing inductance.

As shown in FIG. 6, the resonant inductor Lr and the resonant capacitor Cr are connected in series. When the LLC resonant converter operates at a switching frequency approximately equal to the resonant frequency of the resonant tank, the impedances of the resonant capacitor Cr and the resonant inductor Lr may cancel each other out. As a result, the parallel inductance Lp and the equivalent inductance Lm may be connected in parallel to provide two conductive paths for the magnetizing current.

As described above with respect to FIG. 5, the transformer 152 is so designed that the magnetizing inductance Lm is of an inductance much greater than the parallel inductance Lp. As a result, the majority of the magnetizing current flows through the path provided by the parallel inductance Lp. As such, the parallel inductance Lp is a dominant element during a resonant process in which the magnetizing current is used to discharge the voltages across switching elements so that the switching elements can achieve zero voltage switching or zero current switching. The detailed zero voltage/current switching transitions will be described below with respect to FIG. 7.

FIG. 7 illustrates key switching waveforms of the LLC resonant converter shown in FIG. 5 in accordance with various embodiments of the present disclosure. In order to better illustrate zero voltage switching of the primary side switches and zero current switching of the secondary switches, Q4 and Q8 are selected to illustrate the operation principle of the LLC resonant converter. As shown in FIG. 7, the drain voltage of the primary side switch Q4 is referred to as V_(SW) _(—) _(P). Likewise, the drain voltage of the secondary side switch Q8 is referred to as V_(SW) _(—) _(S).

During one switching period, Q1 and Q4 may be turned on at the same time. The turned on Q1 and Q4, and the resonant tank and the primary side of the transformer winding form a conductive path through which the input voltage source Vin charges the parallel inductor Lp and delivers energy from the primary side to the secondary side through the turned on Q5 and Q8. On the other hand, once Q1 and Q4 are turned off, Q2 and Q3 may be turned on at the same time after a short delay time. The turned on Q2 and Q3, the resonant tank and the primary side of the transformer winding form another conductive path through which the input voltage source Vin resets the parallel inductor Lp and delivers energy to the secondary side through turned on Q6 and Q7.

A switching period is selected to further illustrate the operation principle of the resonant converter. The switching period may start from a first time t1 and stop at a fifth time t5 as shown in FIG. 7. At the first time t1, Q1 and Q4 are turned off. The primary side current Ip charges the drain to source capacitance of Q4. As a result, the voltage V_(SW) _(—) _(P) is charged up to a level approximately equal to the input voltage Vin. Such a voltage level leads to a zero voltage across Q3. At a second time t2, Q3 may be turned on under a zero voltage switching condition. Likewise, Q2 may be turned on at zero voltage.

During the time interval from t1 to t2, as shown in FIG. 7, the current flowing through the parallel inductor Lp is approximately equal to the primary side current Ip. Therefore, the current flowing through the resonant inductor and the resonant capacitor is approximately equal to zero. Such a zero current crossing point of the resonant tank may help to achieve zero current switching of the secondary side switches.

At the second time t2, Q2 and Q3 are turned on through zero voltage switching and Q6 and Q7 are turned on through zero current switching. At a third time t3, both Q2 and Q3 are turned off. Q1 and Q4 are not turned on immediately. Instead, there may be a dead time, which starts from the third time t3 and stops at the fourth time t4. During the dead time, the drain-to-source voltage of Q4 (e.g., V_(SW) _(—) _(P) shown in FIG. 7) is discharged to zero by the current flowing through the parallel inductor Lp. Likewise, the drain-to-source voltage of Q2 is charged up to a level approximately equal to Vin. As a result, at the fourth time t4, both Q1 and Q4 may achieve zero voltage switching.

During the time interval from t4 to t5, Q1 and Q4 are turned on. Energy is delivered from the primary side to the secondary side through the turned on Q5 and Q8. As described above, one advantageous feature of having the LLC resonant converter is the primary side switches may achieve zero voltage switching and the secondary side switches may achieve zero current switching through the dead times (e.g., time intervals from t1 to t2 and from t3 to t4) shown in FIG. 7.

FIG. 8A illustrates another schematic diagram of the LLC resonant converter shown in FIG. 2 in accordance with various embodiments of the present disclosure. The LLC resonant converter 800 is similar to the LLC resonant converter 500 shown in FIG. 5 except that there is a parallel capacitor Cp connected in series with the parallel inductor Lp.

One advantageous feature of having the parallel capacitor Cp is that the LLC resonant converter 800 may achieve a balanced magnetizing flux so that the magnetic failures such as saturation of magnetic components may be prevented.

FIG. 8B illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 8A in accordance with various embodiments of the present disclosure. The equivalent circuit includes two resonant tanks. A main resonant tank is formed by Lr and Cr. An auxiliary resonant tank is formed by Lp and Cp. It should be noted that the resonant frequency of the main resonant tank is greater than the resonant frequency of the auxiliary resonant tank according to some embodiments.

FIG. 9 illustrates a schematic diagram of another LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 900 is similar to the LLC resonant converter 500 shown in FIG. 5 except that the resonant inductance Lr and the resonant capacitor Cr are placed at the secondary side of the transformer. The operation principle of the LLC resonant converter 900 is similar to that of the LLC resonant converter 500 shown in FIG. 5, and hence is not discussed herein to avoid unnecessary repetition.

FIG. 10 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 9 in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 10 is similar to that shown in FIG. 6 except that the parameters of the primary side are reflected to the secondary side. As a result, the equivalent magnetizing inductance may be described as follows:

L _(m) =L _(m) _(—) _(pri) /n ² +L _(m) _(—) _(sec)

where L_(m) _(—) _(pri) is the primary magnetizing inductance and L_(m) _(—) _(sec) is the secondary magnetizing inductance.

FIG. 11 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 1100 is similar to the LLC resonant converter 500 shown in FIG. 5 except that the parallel inductance Lp is placed between the resonant inductance Lr and the resonant capacitor Cr. In other words, Lp is coupled to a common node of the resonant inductor Lr and the resonant capacitor Cr. The operation principle of the LLC resonant converter 1100 is similar to that of the resonant converter 500 shown in FIG. 5, and hence is not discussed herein to avoid unnecessary repetition.

FIG. 12 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 11 in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 12 is similar to that shown in FIG. 6 except that the resonant inductor Lr and the parallel inductance Lp may form a divider circuit, which may cause a gain loss at the output of the LLC resonant converter. In other words, the LLC resonant converter's max gain may be slightly less than 1.

FIG. 13 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 1300 is similar to the LLC resonant converter 500 shown in FIG. 5 except that the resonant capacitor Cr is placed at the secondary side of the transformer. The operation principle of the LLC resonant converter 1300 is similar to that of the resonant converter 500 shown in FIG. 5, and hence is not discussed herein.

FIG. 14 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 13 in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 14 is similar to that shown in FIG. 6 except that the parameters of the primary side is reflected to the secondary side. As a result, the equivalent magnetizing inductance may be described as follows:

L _(m) =L _(m) _(—) _(pri) /n ² +L _(m) _(—) _(sec)

where L_(m) _(—) _(pri) is the primary magnetizing inductance and L_(m) _(—) _(sec) is the secondary magnetizing inductance.

FIG. 15A illustrates another schematic diagram of the LLC resonant converter shown in FIG. 11 in accordance with various embodiments of the present disclosure. The LLC resonant converter 1500 is similar to the LLC resonant converter 1100 shown in FIG. 11 except that there is a parallel capacitor Cp connected in series with the parallel inductor Lp. The advantage of having the parallel capacitor Cp has been described above with respect to FIG. 8A, and hence is not discussed again.

FIG. 15B illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 15A in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 15B is similar to the equivalent circuit shown in FIG. 8B, and hence is not discussed again herein.

FIG. 16 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 1600 is similar to the LLC resonant converter shown in FIG. 11 except that the locations of the resonant capacitor Cr and the resonant inductor Lr are swapped. The operation principle of the LLC resonant converter 1600 is similar to that of the LLC resonant converter 1100, and hence is not discussed again.

FIG. 17 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 16 in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 17 is similar to that shown in FIG. 12. The equivalent magnetizing inductance may be described as follows:

L _(m) =L _(m) _(—) _(pri) +n ² ·L _(m) _(—) _(sec)

where L_(m) _(—) _(pri) is the primary magnetizing inductance and L_(m) _(—) _(sec) is the secondary magnetizing inductance.

FIG. 18 illustrates a schematic diagram of yet another LLC resonant converter in accordance with various embodiments of the present disclosure. The LLC resonant converter 1800 is similar to the LLC resonant converter 500 shown in FIG. 5 except that the resonant inductor Lr is placed at the secondary side of the transformer. The operation principle of the LLC resonant converter 1800 is similar to that of the resonant converter 500 shown in FIG. 5, and hence is not discussed herein

FIG. 19 illustrates an equivalent circuit diagram of the LLC resonant converter shown in FIG. 18 in accordance with various embodiments of the present disclosure. The equivalent circuit shown in FIG. 19 is similar to that shown in FIG. 6 except that the primary side is reflected to the secondary side. As a result, the equivalent magnetizing inductance may be described as follows:

L _(m) =L _(m) _(—) _(pri) /n ² +L _(m) _(—) _(sec)

where L_(m) _(—) _(pri) is the primary magnetizing inductance and L_(m) _(—) _(sec) is the secondary magnetizing inductance

Although embodiments of the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. An apparatus comprising: a resonant inductor coupled to a switching network and a transformer; a resonant capacitor coupled to the switching network and the transformer; a first parallel inductor implemented as a magnetizing inductance; and a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor.
 2. The apparatus of claim 1, wherein: the first inductance is ten times greater than the second inductance.
 3. The apparatus of claim 1, wherein: the second parallel inductor, the resonant inductor and the resonant capacitor form a resonant tank.
 4. The apparatus of claim 3, wherein: the resonant inductor and the resonant capacitor are connected in series and coupled between a first output terminal of the switching network and a first terminal of a primary side winding of the transformer; and the second parallel inductor is coupled between the first output terminal of the switching network and a second output terminal of the switching network.
 5. The apparatus of claim 3, wherein: the resonant inductor and the resonant capacitor are connected in series and coupled between a first output terminal of the switching network and a first terminal of a primary side winding of the transformer; and the second parallel inductor is coupled between a common node of the resonant inductor and the resonant capacitor, and a second output terminal of the switching network.
 6. The apparatus of claim 1, further comprising: a parallel capacitor connected in series with the parallel inductor.
 7. The apparatus of claim 1, wherein: the resonant capacitor is placed at a primary side of the transformer; the second parallel inductor is placed at the primary side of the transformer; and the resonant inductor is placed at a secondary side of the transformer.
 8. The apparatus of claim 1, wherein: the resonant inductor is placed at a primary side of the transformer; the second parallel inductor is placed at the primary side of the transformer; and the resonant capacitor is placed at a secondary side of the transformer.
 9. A system comprising: an input power source; a first pair of switches coupled between the input power source; a second pair of switches coupled between the input power source; a resonant tank connected in series with a primary side of a transformer, wherein: a parallel inductor of the resonant tank is of an inductance less than a magnetizing inductance of the transformer; a first common node of the first pair of switches is coupled to a first terminal of the resonant tank; and a second common node of the second pair of switches is coupled to a second terminal of the resonant tank; a rectifier coupled to a secondary side of the transformer; and an output filter coupled to the rectifier.
 10. The system of claim 9, wherein the resonant tank comprises: a resonant inductor connected in series with a resonant capacitor and coupled between the first common node and a first terminal of the primary side of the transformer; and the parallel inductor coupled between the first common node and the second common node.
 11. The system of claim 9, wherein the resonant tank comprises: a resonant inductor connected in series with a resonant capacitor and coupled between the first common node and a first terminal of the primary side of the transformer; and the parallel inductor coupled between a common node of the resonant inductor and the resonant capacitor, and the second common node.
 12. The system of claim 9, wherein: the transformer is a center-tapped transformer; and the rectifier is a synchronous rectifier.
 13. The system of claim 9, wherein: the transformer is a non center-tapped transformer; and the rectifier is a full-wave rectifier.
 14. The system of claim 9, further comprising: a parallel capacitor connected in series with the parallel inductor.
 15. The system of claim 9, wherein: the first pair of switches, the second pair of switches, the transformer and the rectifier form a full bridge converter.
 16. A converter comprising: a switching network coupled to a dc input power source; a transformer providing isolation between a primary side and a secondary side of the converter; a resonant tank coupled to the switching network, wherein the resonant tank comprises: a resonant inductor coupled to the switching network and the transformer; a resonant capacitor coupled to the switching network and the transformer; a first parallel inductor implemented as a magnetizing inductance; and a second parallel inductor implement as a separate inductor, wherein a first inductance of the first parallel inductor is greater than a second inductance of the second parallel inductor; a rectifier coupled to the secondary side; and an output filter coupled to the rectifier.
 17. The converter of claim 16, wherein: the switching network is configured to operate at a switching frequency approximately equal to a resonant frequency of the resonant tank.
 18. The converter of claim 16, wherein: the resonant tank further comprises a parallel capacitor connected in series with the parallel inductor.
 19. The converter of claim 16, wherein: the resonant inductor connected in series with a primary winding of the transformer; and the resonant capacitor connected in series with a secondary winding of the transformer.
 20. The converter of claim 16, wherein: the resonant capacitor connected in series with a primary winding of the transformer; and the resonant inductor connected in series with a secondary winding of the transformer. 